1. Field of the Invention
This invention relates, in general, to an improved catalyst system and a process for using the improved catalyst system and, specifically, to a catalyst system of a metallocene compound with a Lewis acid and an aluminum alkyl and a process for using the improved catalyst system for polymerization of olefins.
2. Description Of Related Art
The use of metallocene compounds as catalysts for the polymerization of olefins is known. German patent application No. 2,608,863 discloses a catalyst system for the polymerization of ethyl consisting of bis(cyclopentadienyl) titanium dialkyl, an aluminum trialkyl and water. German patent application No. 2,608,933 discloses an ethylene polymerization catalyst system consisting of zirconium metallocenes of he general formula (cyclopentadienyl).sub.n ZrY.sub.4-n, wherein Y represents R.sub.1 CH.sub.2 AlR.sub.2, CH.sub.2 CH.sub.2 AlR.sub.2 and CH.sub.2 CH(AlR.sub.2).sub.2 where R stands for an alkyl or metalloalkyl, and n is a number within the range 1-4; and the metallocene catalyst is used in combination with an aluminum trialkyl cocatalyst and water.
The use of metallocenes as catalysts in the copolymerization of ethylene and other alpha-olefins is also known in the art. U.S. Pat. No. 4,542,199 to Kaminsky et al. discloses a process for the polymerization of olefins and particularly for the preparation of polyethylene and copolymers of polyethylene and other alpha-olefins. The disclosed catalyst system includes a catalyst system of the formula (cyclopentadienyl).sub.2 MERHal in which R is a halogen, a cyclopentadienyl or a C.sub.1 -C.sub.6 alkyl radical, Me is a transition metal, in particular zirconium, and Hal is a halogen, in particular chlorine. The catalyst system also includes an alumoxane having the general formula Al.sub.2 OR.sub.4 (Al(R)--O).sub.n for a linear molecule and/or (Al(R)--O).sub.n+2 for a cyclic molecule in which N is a number for 4-20 and R is a methyl or ethyl radical. A similar catalyst system is disclosed in U.S. Pat. No. 4,404,344.
U.S. Pat. No. 4,530,914 discloses a catalyst system for the polymerization of ethylene to polyethylene having a broad molecular weight distribution and especially a bimodal or multimodal molecular weight distribution. The catalyst system is comprised of at least two different metallocenes and an alumoxane. The patent discloses metallocenes that may have a bridge between two cyclopentadienyl rings with the bridge serving to make those rings stereorigid.
European Patent Publication No. 0185918 discloses a stereorigid, chiral zirconium metallocene catalyst for the polymerization of olefins. The application does not indicate that hafnium could be substituted for the zirconium and used to produce a useful polymer product. The bridge between the cyclopentadienyl groups is disclosed as being a linear hydrocarbon with 1-4 carbon atoms or a cyclical Hydrocarbon with 3-6 carbon atoms.
Polymerization of olefins is primarily with Ziegler-Natta catalysts. One family of Ziegler-Natta catalysts is Group IV metallocene compounds with methylaluminoxane (MAO) as a cocatalyst. A system for the production of isotactic polypropylene using a titanium or zirconium metallocene catalyst and an alumoxane cocatalyst is described in "Mechanisms of Stereochemical Control in Propylene Polymerization with Soluble Group 4B Metallocene/Methylalumoxane Catalysts," J. Am. Chem. Soc., Vol. 106, pp. 6355-64, 1984. The article shows that chiral catalysts derived from the racemic enantiomers of ethylene-bridged indenyl derivatives form isotactic polypropylene by the conventional structure predicted by an enantiomorphic-site stereochemical control model. The meso achiral form of the ethylene-bridged titanium indenyl diastereomers and achiral zirconocene derivatives, however, produce polypropylene with a purely atactic structure.
MAO activates the metallocene which then becomes able to promote polymerization of alpha-olefins. Other organometallic compounds of aluminum, such as trimethyl aluminum (TMA) or dimethyl aluminum halide, are known not to promote polymerization by themselves. Neither aluminum alkyls nor dimethylaluminum halides alone are active cocatalysts.
A combination of TMA and dimethylaluminum fluoride (DMF) has been shown to be effective as a cocatalyst in place of MAO. DMF is a Lewis acid. Such a combination is reported in "Isotactic Polymerization of Propene: Homogeneous Catalysts Based on Group 4 Metallocenes without Methylalumoxane", A. Zambelli, P. Longo and A. Grassi, Macromolecules, Vol. 22, p. 2186-2189, 1989. The stereochemical structure of the polymers prepared with TMA/DMF and with MAO were very similar. However, the polymerization yields obtained for TMA/DMF mixtures were substantially lower than those obtained for MAO.
It has also been reported that a metallocene compound with magnesium chloride forms a catalyst system with bis(cyclopentadienyl)thorium dimethyl which is effective to polymerize ethylene. Such a combination is reported in "(CH.sub.2)C.sub.5 !.sub.2 Th(CH.sub.3).sub.2 Surface Chemistry and Catalysis. Direct NMR Spectroscopic Observation of Surface Alkylation and Ethylene Insertion/Polymerization on MgCl.sub.2 ", D. Heddin and T. J. Marks, J. Am. Chem. Soc., Vol. 110, No.5, 1988. A methyl group is abstracted from the cyclopentadienyl compound and a cation is formed. The methyl group coordinates with the magnesium anion. Magnesium chloride is a Lewis acid.
Metallocene catalysts are sensitive to poisons in the absence of a scavenging agent, such as methylaluminoxane. Polymerization requires high concentrations of the cations and frequently end up as either runaway reactions or yield no polymer at all.